package com.hjx.leetcode;

import java.util.EnumSet;

/**
 * 53. 最大子序和
 * 给定一个整数数组 nums ，找到一个具有最大和的连续子数组（子数组最少包含一个元素），返回其最大和。
 *
 * 示例:
 *
 * 输入: [-2,1,-3,4,-1,2,1,-5,4],
 * 输出: 6
 * 解释: 连续子数组 [4,-1,2,1] 的和最大，为 6。
 * 进阶:
 *
 * 如果你已经实现复杂度为 O(n) 的解法，尝试使用更为精妙的分治法求解。
 *
 * 参考： https://www.rongsoft.com/article/2020/02/200937334557/
 */
class LeetCode_53 {

    public int maxSubArray(int[] nums) {
        int maxSum = nums[0];
        for (int i = 0; i < nums.length; i++) {
            int tempMaxSum = getMaxSum(nums, i);
            if (tempMaxSum > maxSum) {
                maxSum = tempMaxSum;
            }
        }
        return maxSum;
    }

    private int getMaxSum(int[] nums, int i) {
        int max = nums[i];
        int sum = 0;
        for (; i != -1 ; i--) {
            sum += nums[i];
            if (sum > max) {
                max = sum;
            }
        }
        return max;

    }

    public int maxSubArray2(int[] nums){
        int[] dp = new int[nums.length];
        int maxSum = nums[0];
        for (int i = 0; i < nums.length; i++) {
            int tempMaxSum = getMaxSum(dp, nums, i);
            dp[i] = tempMaxSum;
            if (tempMaxSum > maxSum) {
                maxSum = tempMaxSum;
            }
        }
        return maxSum;
    }

    private int getMaxSum(int[] dp, int[] nums, int i) {
        int max = nums[i];
        int sum = 0;
        dp[i] = dp[i-1] + nums[i];

        for (; i != -1 ; i--) {
            sum += nums[i];
            if (sum > max) {
                max = sum;
            }
        }
        return max;

    }
}
